This invention relates in general to epoxy resins and, more specifically, to amine reacted epoxy resin blends particularly useful in fiber reinforced composites with initial curing by low energy radiation and thermal final cure.
A great many different epoxy resin formulations, resin mixtures and methods of making epoxy resins have been developed over the years for a variety of different purposes and to manufacture many different products. Generally, these formulations have been optimized for the manufacture of products to be cured by exposure to elevated temperatures. Epoxy resins, for example, have come into widespread use as the matrix resin for fiber reinforced composite structures of many kinds.
Present methods for manufacturing products from resin impregnated fiber material generally arrange multiple layers of resin impregnated tow or fabrics on a tool surface, then use some method of applying uniform pressure to the laid-up material while heating the assembly to cure the resin. Typical methods of applying pressure include vacuum bagging, multiple staging and compaction operations, autoclave curing, closed die molding, etc. These methods are quite effective with reasonably small and thin products. However, they are less successful with large and/or thick products.
With large or thick products, the ability to cause volatile gases generated by the matrix resin to migrate along preferred pathways transverse to the fiber lengths into a breather/bleeder cloth or through vent ports becomes very difficult. Strength reducing large voids and general porosity occur in the final product if sufficient outgassing of these volatile gases is not accomplished.
In order to achieve uniform physical properties in a composite product, it is necessary that the resin distribution throughout the product be uniform. Resin migration or flow in lay-ups where the resin is liquid tends to be dominated by fiber tension and the preferential flow direction. A consequence is the tendency to have significantly lower resin contend fibers at the interior layers and higher than desirable resin content in the outer layers, resulting in lower composite strength and varying physical properties.
Thermal curing of thermosetting resins through cationic polymerization reactions have generally been used in the past. These require prolonged dwell times at relatively high temperatures to produce cross-linking and final curing of the matrix resin. The great amount of time required results in poor production efficiency. Controlling this type of reaction with large and/or thick structures is difficult due to the exothermic behavior common to thermosetting resins. These problems are most often dealt with by accepting extremely lengthy cure cycles which use very slow heat up rates and several constant temperature dwells. Total cure times in excess of 12 hours are not at all uncommon for very large and/or thick thermoset resin matrix composite structures.
Expensive high temperature capable tooling is required to withstand the processing heat and pressures required in these prior processes. Most often this requires expensive metal tooling which can be difficult to machine, heavy and creates a large thermal mass that must be accounted for in the cure cycle development.
In some cases, thermoplastic matrix materials are used in fiber reinforced composites. Processing times for thermoplastic resins can be relatively short, since they only require heating beyond their melting temperatures with consolidation pressure to cause complete laminate consolidation. However, in many cases the processing temperature are quite high, requiring more expensive and complex tooling. In addition, most of the advanced thermoplastic matrix materials are much more expensive that equally performing thermosetting resin matrix materials.
Radiation curing of resin matrix composite parts has been used in some instances. These processes are intended for the polymerization and curing of fully laid up composite material in its final size and thickness through the use of very high energy electron beam radiation sources, typically around 20 kW , 10 MeV and X-rays in doses up to 10 Mrad. These processes rely on very high energy radiation to penetrate great distances into the composite structure, up to about 12 inches, to effect complete curing of the part in its entirety. Such high energy radiation sources are very costly to design and construct. Containment of the radiation requires isolating the radiation sources behind very thick, often greater than six feet thick, concrete walls. The entire assembly of tooling and composite material must be moved into the chamber for curing.
Different thermosetting resins and different end uses for composite structures may require different polymerization and curing methods. Cryogenic applications, such as liquid hydrogen and oxygen containers for space launch vehicles, containers, piping and the like for superconducting magnet cooling systems using liquid helium or liquid nitrogen require composites having acceptable physical properties at extremely low temperatures. Prior methods and materials for use in fabricating structures for use at cryogenic temperatures have not been fully satisfactory.
Thus, there is a continuing need for improved resin systems and methods of polymerizing and curing the resin matrices in fiber reinforced composite manufacture which can employ high energy radiation, such as electron beam or ultraviolet radiation to initiate cure to a shape retaining state, use simple, inexpensive tooling, produce a final cure by thermal means, reduce voids and other variations in the product and produce structures useful under a variety of conditions.